Drug addiction is a psychiatric disorder characterized by a transition from recreational to compulsive drug use that continues in spite of severe negative consequences. Despite attempts by individuals to quit, the desire or need to resume drug-taking can last for months or years. The persistence of addiction over time suggests that exposure to drugs results in long-term adaptations in the brain that likely involve alterations in transcription and genetic regulation. In addition to genetic factors, epigenetic mechanisms may also play a role in the maintenance of addictions not only throughout an individual's lifetime, but in his/her descendants. However, the potential impact of drug exposure across generations has not been characterized. To date, no comprehensive study has been undertaken to determine if exposure to drugs of abuse in the parental generation leads to alterations in behavioral or molecular phenotypes in subsequent generations. To address this question, we will examine the phenotypic consequences of two mechanistically different drugs (cocaine and morphine) in three inbred mouse strains (C57BL/6J, DBA/2J, A/J) as well as a multi-generation series of F1 hybrids. In addition to studying behavioral phenotypes across generations, we will characterize RNA expression, DNA methylation, and post-translational histone modifications as both the molecular signature and the mechanistic basis for heritable epigenetic changes. These studies will allow us to determine (1) whether any of 2 mechanistically different drugs of abuse results in transmission of addiction-related phenotypes through multiple generations after treatment, (2) whether these phenotypes are mediated by changes to the epigenome (DNA methylation, post-translational histone modifications, or RNAs), and (3) whether these phenotypes vary according to the sex of exposed parent in an imprinted or allele-specific manner. These experiments will inform future studies aimed at elucidating the mechanisms by which drugs of abuse lead to transgenerational phenotypes.